Usually pressure-measuring cells have four deformation-dependent resistance elements in a full bridge circuit, wherein two first resistance elements have a first deformation-dependence and two second resistance elements have a second deformation-dependence. The resistance elements are alternately arranged in a closed bridge circuit, which is fed via a first diagonal, and whose deformation-dependent signal is tapped via the second diagonal of the bridge circuit.
Resistance elements include, for example, piezoresistive, semiconductor resistors, which are produced by local doping in a base material, silicon for example. Insofar as doping is associated with concentration gradients, such resistance elements and/or their deformation-dependence can have a tendency to change, especially at high temperatures, since the concentration gradients are unstable due to diffusion phenomena.
There is thus, in principle, a need for pressure sensors made of other materials, which do not exhibit the mentioned doping-related problems.
Eickhoff et al. have, in the Journal of Applied Physics, Vol. 90, No. 7, described the piezoresistivity of AlxGa1−xN layers with wurtzite structure and AlxGa1−xN/GaN heterostructures. In this work, the experimental results are interpreted as regards deformation-dependent resistances supportably with the model of a mechanically controlled gate of a field effect transistor, whose resistance value depends on the deformation-dependent piezoelectric field along the axis perpendicular to the plane of the AlxGa1−xN layers.
It is to be taken further from this work that an Al concentration between, for example, 0 and 0.35 is suitable as a parameter for the variation of the deformation-dependence of an electrical resistance.
Stutzmann et al. have, in Phys. Stat. Sol. (b) 228, No. 2, 505-512 (2001), described the influence of the polarity of AlxGa1−xN layers, as a function of terminations with N, respectively Ga or Al, on the electrical characteristics of the layers. It appears, to that extent, interesting to take up the materials described in the above works for implementing a pressure sensor.